FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesMedium
A technician performs a Rockwell hardness test on a steel bolt to verify heat treatment. Which principle does this test use to determine hardness?
- ADepth of penetration of an indenter under a fixed load
- BResistance to scratching by materials of known hardness
- CRebound height of a diamond hammer dropped onto the surface
- DTime required for an indenter to fully penetrate the surface
Show answer & explanationAnswer & explanation
Correct answer: A. Depth of penetration of an indenter under a fixed load
The Rockwell hardness test applies a minor load followed by a major load through an indenter (diamond cone or steel ball) and measures the depth of penetration into the material; harder materials show less penetration depth, which corresponds to a higher Rockwell hardness number. This differs from the Shore scleroscope test (rebound) and the Mohs scale (scratch resistance).
Why the other options are wrong
- B. Scratch resistance describes the Mohs hardness scale, used for minerals, not metals in aviation testing.
- C. Rebound height describes the Shore scleroscope test, not the Rockwell method.
- D. Rockwell testing does not measure time to penetrate; it measures depth under a fixed load.
Rockwell Hardness Test
A hardness test that measures the depth of penetration of an indenter into a material under a fixed major load after a minor preload, with shallower penetration indicating greater hardness.
- Uses a diamond cone or hardened steel ball indenter
- Applies a minor load first, then a major load
- Hardness value is derived from the difference in penetration depth
Memory trick: 'Rockwell digs deep to measure hard' - depth of dent tells the story